This invention relates to reagents useful as inhibitors of hepatitis C virus (HCV) replication and gene expression.
The following is a discussion of relevant art, none of which is admitted to be prior art to the pending claims.
The Hepatitis C virus group is responsible for most cases of non-A, non-B hepatitis. Although non-A, non-B hepatitis may be caused by many different viruses, the principal cause is HCV. The infection is transmitted through blood transfusions (25-50% of hemophiliacs have chronic hepatitis), percutaneous infusion from illicit drug usage (25-50% of all drug users have non-A, non-B hepatitis), renal organ transplant (where most late mortality is due to non-A, non-B hepatitis), and maternal-neonatal transfer. Up to 20% of sporadic outbreaks of hepatitis are caused by HCV.
The association of hepatitis C virus in posttransfusion hepatitis (&gt;95% of the cases) and the subsequent observation that infection with this virus, more so than with hepatitis B virus, correlates strongly with the development of hepatocellular carcinoma and liver cirrhosis has led to intense study of the molecular biology of the viral genes.
Hepatitis C virus (HCV) is a positive-stranded, enveloped virus which is evolutionarily related to the flaviviruses and pestiviruses. The HCV genome consists of a 5' untranslated region (5' UTR), a long open reading frame of between 9030 and 9099 nucleotides and a 3' UTR. The 3' end of the genome has been reported to contain a short stretch of poly(A) in type I strains, but this has not been confirmed in type II, III or IV strains. It has been reported that sequences in the 5' UTR exhibit negative translational control over viral gene expression, but the major regulation of HCV gene expression comes from the proteolytic clevages of the polyprotein. Various proteolytic cleavages of the polyprotein are essential for the efficient expression of replication-associated viral proteins after translation. The resultant replicase proteins interact with a region of between 27 and 45 nucleotides in the 3' UTR which functions as a structural cue for the initiation of viral genomic replication.
Cleavage of the polyprotein of HCV uses both cellular and virally-encoded proteases and generates at least nine proteins. A tenth viral protein (a second protease) has been reported to be generated by alternative cleavage of the polyprotein, but has not been confirmed. Confirmed virally-encoded proteins can be classified as structural or non-structural proteins. The structural proteins of the virus are the C protein (22 kD), E glycoprotein (35 kD) and the E2/NS1 glycoprotein (58 kD). The non-structural proteins include the NS2 (53 kD), NS3 (70 kD), NS4a (8 kD), NS4b (27 kD), NS5a (58 kD) and NS5b (68 kD).
The C protein is a hydrophobic protein which constitutes the core structural component of the viral particle and may play a role in localizing the viral particle to the cellular membrane. There is also evidence from transient assays that the core protein may function as a transactivator of gene expression. This protein is essential to viral replication and appears to be highly conserved at the nucletide level of homology among known types of HCV.
The envelope proteins are necessary for viral replication but their nucleotide (and amino acid) sequences diverge significantly between types of HCV. The NS3 protein functions as the major viral protease and mutations in this protein lead to inactivation of viral replication by inhibiting the processing of the polyprotein. The nucleotide sequence of the NS3 region is highly conserved among viral isolates.
The level of infidelity exhibited by the HCV RNA replicase has been indirectly quantified by assessing the sequential appearance of nucleotide substitutions in the genomes of HCV samples obtained from the same patient. These studies suggested that the nucleotide divergence was minimal but it may have been an underestimate of the actual infidelity of the replicase because the recovered samples were actually pre-selected by growth in the patient. Unacceptible substitutions would have been lost during the outgrowth of the virus.
The replication of HCV in hepatocytes is integrally intertwined with the replication of other hepatitis viruses. Although hepatitis delta virus (HDV) is quite distinct from HCV, the life cycles of the two viruses can be interdependent. Normally HDV depends upon the replication of Hepatitis B Virus (HBV) to supply capsid proteins and other functions necessary for the assembly of infectious HDV particles. Because HCV inhibits HBV and Hepatitis A virus (HAV) growth in co-infected cells, the replication of HCV can also inhibit the maturation of HDV. The use of HDV as a gene vector for treatment of HCV infections would not be influenced by this interference because gene expression of the HDV genome is apparently uninhibited by HCV.